Relationship between paper smoothness and printing and its measuring instruments (2)

1. Smoothness and number of screens

The number of screen lines is the number of Internet sites per unit length. It is usually expressed in terms of how many lines per inch or lines per cm. The center distance between every two dots is equal to the reciprocal of the dot lines. For offset printing, the higher the number of lines and the same percentage of outlets, the smaller the number of outlets, the finer the level of performance, and the stronger the ability to express levels, but the more serious the network points are in the printing process; the lower the number of lines, With the same percentage of outlets, the larger the outlets are, the worse the performance of the sub-levels is, but the outlets in the printing process are lighter. The number of screens commonly used in printing is 80, 100, 120, 133, 150, 175, 200, 300 lpi, etc. The number of screens for different papers is shown in Table 1.

In actual selection, the surface properties of the paper are an important factor. The surface properties such as the smoothness and roughness of the paper determine that they have different requirements for the number of screens. Coated paper or high-calendered white paperboard has a high surface smoothness and can reproduce finer dots. Therefore, the number of screen lines is relatively high, generally set at 175 to 300 lpi. The surface of offset paper is rougher than coated paper, and its number of screens can be up to 120. ~150lpi; newsprint surface is more rough, too small dots will form a broken edge, or completely fall in the sunken place, it should use a larger dot printing, the number of screen lines can be 80 ~ 133lpi.

2. Smoothness and dot reproduction

The smoothness of the paper affects the reproduction and enlargement of the dot. Paper with a high degree of smoothness has a good reproducibility of its outlets, and its dot gains are not serious. Because the use of paper with lower smoothness, under the same printing pressure conditions, the contact area between the paper and printing plate or blanket is small, affecting the transfer of graphic information, in order to complete the transfer of graphic information in the actual situation. Most of them are achieved by increasing the printing pressure and the amount of ink, and this will cause a serious increase in the number of outlets and affect the quality of the printed products; moreover, when using a paper with a low degree of smoothness, it is difficult to achieve a fine dot. Reproduction.

3. Smoothness and ink transfer

Paper smoothness affects the demand for printing inks. The smaller the paper smoothness, ie, the greater the roughness, the greater the amount of ink needed to achieve the same printed surface density. Although the increase in the amount of ink to achieve the desired effect of the surface density, but due to the uneven surface of the paper, the resulting print surface is less uniform, and easy to appear through the phenomenon of printing, seriously affecting the quality of printed products. In addition, the smoothness of the paper also has a great influence on the uniformity of the ink transfer and the gloss of the surface of the print. Paper with high smoothness tends to form a smooth ink film and increase the gloss of the surface of the print.

4. Smoothness and color effect

The smoothness of paper is an important factor in determining the quality of printed color reproduction. Generally speaking, the paper with high smoothness can transfer the ink evenly at the contact point between the graphic and the dot with the largest contact area during embossing, so that the dots can be clearly and fully reproduced on the paper, and thus the color of the ink can be Gloss is fully reflected on the surface of the paper. On the other hand, paper with low smoothness has uneven and insufficient ink transfer, and it has a strong penetrability to the ink on the paper surface, resulting in a poor color rendering effect, and tends to cause the printed ink to fade, fade, and partially print.